White blood cells help to defend against pathogens by:

The idea

White blood cells are part of the body's second line of defence, acting after physical and chemical barriers such as skin and mucus. They circulate in blood and tissue fluid and respond to pathogens that have entered the body. Defence occurs by two main mechanisms: phagocytosis, in which a white blood cell engulfs and digests a pathogen, and antibody production, in which lymphocytes release antibodies that bind to antigens on a pathogen, marking it for destruction or neutralising it. Some white blood cells also produce antitoxins that neutralise toxins released by bacteria. This statement introduces the general role of white blood cells; the named processes are covered in the following statements. A concrete example is a cut becoming infected: bacteria entering through the break are met by white blood cells that engulf them and by antibodies that clump them together.

What earns the marks

• White blood cells are a component of the blood and defend the body against pathogens that have entered it.
• Phagocytosis is one defence mechanism: a white blood cell engulfs a pathogen and digests it.
• Antibody production is another defence mechanism: white blood cells release antibodies that bind to antigens on pathogens.
• Antibodies can cause pathogens to clump together or be destroyed, and some white blood cells produce antitoxins that neutralise bacterial toxins.
• White blood cells act alongside, not instead of, the body's physical and chemical barriers such as skin and mucus.

Common mistakes and how to put them right

• Thinking that white blood cells only carry oxygen.

  Put it right: Red blood cells carry oxygen, while white blood cells defend against pathogens.

• Believing that white blood cells physically block pathogens at the body surface.

  Put it right: Physical barriers such as skin do that, whereas white blood cells act once pathogens have entered.

• Assuming all white blood cells work in exactly the same way.

  Put it right: Different white blood cells carry out phagocytosis, antibody production or antitoxin production.

Exam tips

• Name the two main mechanisms, phagocytosis and antibody production, and link each to a clear outcome for the pathogen.
• Use the terms pathogen, antigen and antibody accurately; an antigen is on the pathogen, while an antibody is produced by the white blood cell.
• When a question asks how white blood cells defend the body, give a mechanism and its effect rather than only naming the cell type.